PRODUCTION OF OLEFINS

Abstract: A process for cracking or decomposing a feedstream containing a major proportion of at least dialkyl ether to produce the corresponding olefins comprises contacting the feedstream with a faujasite aluminosilicate catalyst, wherein at least about 50 wt.% of the alkali metal content orginally present in said faujasite has been exchanged by at least one alkaline earth metal, characterized in that prior to the contacting the catalyst is pretreated with steam which enhances its selectivity and reduces the undesirable by-product yields.

Description: PRODUCTION OF OLEFINS The present invention relates to a process for the production of olefins. More particularly, but not exclusively, it relates to a method for the production of pure tertiary olefins by the decomposition of alkyl tert-alkyl ethers in the presence of a new and improved catalyst based on an alkaline earth exchanged faujasite. Additionally, the selectivity of the new and improved catalyst is enhanced and the undesirable by-product yields are reduced, by pretreating the catalyst with steam. Olefins, particularly tertiary olefins, may be commercially produced by the sulfuric acid extraction of such olefins from mixtures containing them obtained e.g., by steam cracking of petroleum feeds. Since this method uses sulfuric acid of high concentration, the use of expensive materials in the fabrication of the extraction apparatus is essential. Also, dilution of the acid to promote olefin recovery and reconcentrating the acid prior to recycling are required and are expensive. In addition, this method is not always advantageous industrially because tertiary olefins are subject to side reactions such as polymerization, hydration and the like during extraction with concentrated sulfuric acid. It is also known that tertiary olefins may be prepared by reacting them selectively from such feeds with a primary alcohol in the presence of an acid catalyst to produce the corresponding alkyl tert-alkyl ethers. The tert-alkyl ethers are primarily formed, since the secondary olefins react very slowly and the primary olefins are completely inert. Such alkyl tert-alkyl ethers may then be easily separated and subsequently decomposed back to the tertiary olefins and the primary alcohol. For producing tertiary olefins from alkyl tert-alkyl ethers, there have been proposed methods using various catalysts: For example aluminum compounds supported on silica or other carriers (US-A-4,398,051); phosphoric acid on various supports (US-A-4,320,232); and metal containing weakly acidic components on a carrier of >20 m2/gm surface area (GB-A- 1,173,128). In addition, inferior results are disclosed as being obtained utilizing carriers alone in the decomposition of methyl tertiary butyl ether (US-A-4,398,051) and utilizing H2SO4 treated clay in the decomposition of t- alkyl ether alkanols (US-A-4,254,290). One of the main disadvantages of such processes is that the disclosed catalysts do not have good catalyst life in that higher and higher temperatures, which eventually become limiting, are required to maintain high conversion of the alkyl tert-alkyl ethers. Additionally, larger amounts of the dialkyl ether by-product are produced as the catalyst ages with the disadvantage of a reduction in yield of the desired tertiary olefin. This lack of good catalyst life may be due to the instability of the catalyst, to high temperatures being required for good conversion thus promoting fouling, to the catalyst itself promoting fouling or to any or all of these. Also, a number of the catalysts such as ion exchange resins cannot be regenerated after use. More recently, processes have been discovered which provide improved yields of tertiary olefin product. For example, US-A-4,691,073 discloses a process for preparing tertiary olefins from alkyl tertiary alkyl ethers comprising contacting the ether with a catalyst which has been prepared by reacting a clay with HF and/or HC1 and calcining the resultant clay product. Although the process produces very high yields and selectivity towards the production of tertiary olefin products, these catalysts often tend to become deactivated as a consequence of coke and/or polymeric build up in a relatively short on-stream time. Also, the aluminosilicate structure of many clays is not sufficiently stable to withstand repeated high temperature regenerations required to remove catalyst deposits. Natural and synthetic faujasite catalysts are known for use in the conversion or pyrolysis of ethers and alcohols into olefins or distillate range hydrocarbons. For example, US-A-4,467,133 (Chang) discloses the conversion of methanol into a distillate range hydrocarbon mixture by passing methanol over a rare earth exchanged faujasite (such as zeolite X or Y) at a temperature below 315° C (600° F). More particularly, Chang discloses a process for converting lower alcohols (Cj to C4 alcohols) or lower dialkyl ethers to distillate range (CJQ + ) hydrocarbons suitable for use as diesel fuels. The process involves passing the feedstream over one of numerous disclosed aluminosilicate catalysts the alkali metal content of which has been exchanged with hydrogen or a Group IB - VIII metal. The focus of the disclosure is on zeolite Y as the preferred zeolite and a rare earth metal as the preferred exchange action. Example 1 on column 6 shows the conversion of methanol to CIQ-C29 + olefins using REY zeolite. Quite clearly the process leads to dehydrogenation and oligomerization of the components in the feed streams, as evidenced by conversion of methanol to olefins having a minimum of 10 carbon atoms. This is quite distinct from the process of the present invention, where an alkaline earth metal exchanged faujasite, e.g., zeolite Y, is used as the catalyst which does not produce significant amounts of distillate range hydrocarbons (as does Chang) but rather selectively converts alkyl ethers to their corresponding olefins. US-A-5,254,785, equivalent to co-pending PCT application, PCT/US93/05399, teaches selectivity improvement in the process of converting an alkyl ether to its corresponding olefin using a catalyst based on an alkaline earth exchanged faujasite with absolutely no mention of steam. It would be desirable to be able to further improve the selectivity of this catalyst, while at the same time reduce the undesirable by-product make, when using this catalyst to make olefins. By adding steam to pretreat the catalyst, additional selectivity improvements are achieved with reduced by¬ product make. SUMMARY OF THE INVENTION According to the present invention, there is provided a process for selectively converting an alkyl ether to its corresponding olefin which comprises contacting the ether with a faujasite catalyst wherein at least 50% by weight of the original alkali metal content has been exchanged with at least one alkaline earth metal, characterized in that prior to the contacting, the catalyst is pretreated with steam to enhance catalyst selectivity and to reduce the undesirable by-product yields. The alkyl ether may be one having for example at least 5 carbon atoms, preferably from 5 to 12 carbon atoms and more preferably from 5 to 8 carbon atoms. The process is particularly but not exclusively suited to conversion of tertiary alkyl ethers, such as tertiary butyl methyl ether or tertiary amyl methyl ether, to their corresponding olefins. In accordance with the invention, one or a mixture of ethers may be converted to their corresponding olefins. The ether(s) may comprise a component of a feed which is contacted with the specified catalyst. Thus, the invention also provides a process for cracking or decomposing a feedstream containing a major proportion of at least one dialkyl ether having at least about 5 carbon atoms to produce the corresponding olefins. In accordance with the invention, the catalyst is pretreated with steam until the catalyst is saturated. Typically the steaming is conducted at a temperature from 100 to 400 °C (212 to 752° F), preferably from 100 to 232° C (212 to 450 °F), and most preferably to 121 to 177° C (250 to 350° F), at a space velocity of from 0.1 to 15 hr 1 WHSV, preferably from 0.3 to 5 hr 1 WSHV, and most preferably from 0.5 to 3 hr"l WHSV, and at a pressure of from atmospheric pressure to 4140 kPag (600 psig), preferably from atmospheric pressure to 1720 kPag (250 psig), and most preferably 103 to 1030 kPag (15 to 150 psig). The conversion, cracking or decomposition is preferably conducted in the vapor phase. The preferred temperatures for the contact between ether and catalyst are in the range of from 51° to 315° C (125° to 600° F). The process generally offers the advantages of longer catalyst life coupled with high yield and selectivity rates towards production of the olefin which corresponds to the starting ether. Additionally, the production of undesirable by-products, such as methyl secondary butyl ethers, dimethyl ether, and isobutane, is reduced, by at least 20%, preferably 30%, and most preferably 40%. The contact between the ether and the catalyst is generally performed under conditions of temperature and pressure sufficient to convert a substantial proportion of the ether to its corresponding olefin. Rates of conversion of ether to olefin in excess of 60% by weight may be obtained, preferably in excess of 90% by weight. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a plot of the effective CaY catalyst activity with (the invention) and without (comparative) pretreatment with steam. Figure 2 is a plot of the corresponding methyl secondary butyl ethers (%) conversion with and without pretreatment with steam using CaY catalyst. Figure 3 is a plot of the corresponding dimethyl ether (mol %) with and without pretreatment with steam using CaY catalyst. Figure 4 is a plot of the corresponding isobutane levels (wt. ppm) with and without pretreatment with steam using CaY catalyst. Figure 5 is a plot of the effective comparative ZSM5 catalyst activity with and without pretreatment with steam. Figure 6 is a plot of the methyl secondary butyl ethers (%) conversion with and without pretreatment with steam using comparative ZSM5 catalyst. Figure 7 is a plot of the dimethyl ether (mol %) with and without pretreatment with steam using comparative ZSM5 catalyst. Figure 8 is a plot of the isobutane levels (wt. ppm) with and without pretreatment with steam using comparative ZSM5 catalyst. DETAILED DESCRIPTION OF THE INVENTION The catalyst which may be used in the process of this invention is based on a natural or synthetic faujasite, for example zeolite Y. This is an alkali metal containing crystalline aluminosilicate well known in the art and is described in US- A-3, 130,007. The preferred faujasite has a silica to alumina molar ratio in the range of from 3 to 1 to 6 to 1 and/or pore dimensions greater than about 6 Angstroms. This zeolite material may be activated for the ether decomposition (conversion) reaction by base exchanging the alkali metal originally present in the zeolite, e.g. sodium, with one or a mixture of alkaline earth metals such that at least 50% by weight of the alkali metal is replaced by the alkaline earth metal. It is preferred to conduct the exchange such that as many as possible of the original alkali metal ions are so exchanged, e.g., at least 75% by weight and more preferably at least 85% by weight. Most preferably the exchange is such that the original alkali metal content of the zeolite is reduced to a level below about 1% by weight and the degree of exchange is about 90% or above. Suitable alkaline earth exchange metals are calcium, barium and strontium, with calcium being most preferred. Base exchange may be conducted for example by contacting the zeolite (which has been preferably previously calcined) one or more times with an aqueous solution containing an alkaline earth metal salt dissolved therein, preferably at a temperature ranging from ambient up to about 85° C (185°F). A wide variety of salts may be employed such as the chlorides, bromides, carbonates, sulfates or nitrates, so long as such salts are soluble in water such that ion transfer can take place. Calcium chloride is the preferred salt. The concentration of the salt in solution may for example range from about 0.1 to about 25% by weight. Preferably the concentration is sufficient to provide a slight excess of the stoichiometric amount of exchange cation. After an exchange contact period which may range for example from about 60 minutes to about 24 hours, the exchanged zeolite is separated from the exchange solution, washed and dried. The exchange can be repeated one or more times if necessary in order to replace the maximum number of alkali metal ions with alkaline earth metal ions. Other exchange processes may also be employed, such as the so called incipient wetness method, wherein the zeolite is infused with exchange solution to form a paste which is then dried. The catalyst may be used in the process without additional binder or it may be formulated with a binder or carrier material such as alumina, silica, clay or an alumina/silica mixture. Bound catalyst may be prepared by mixing the powdered catalyst with water and preferably from 5 to 40% by weight binder to form a paste, and extruding and drying the paste to form small pellets. The bound catalyst is then preferably further activated by calcination, 343°-593°C (650 100°F) and preferably for a period of about 10 minutes up to a period of hours, e.g., 24 hours. The ion exchange process may be conducted prior to or subsequent to the formulation of such bound zeolites, preferably subsequent to such formulation. Ethers which may be cracked (converted) using the specified catalyst in the process of this invention preferably contain from 5 to 12 carbon atoms, more preferably from 5 to 9 carbon atoms and most preferably from 5 to 8 carbon atoms. Preferred ethers include tertiary alkyl ethers such as tertiary butyl methyl ether and tertiary butyl ethyl ether, and tertiary amyl counterparts including the methyl and ethyl ethers. Feedstreams which may typically be employed in commercial applications of the process preferably contain at least 70% up to 100% by weight of the ether, for example tertiary alkyl ether. The balance (if any) of the feedstream may comprise for example, primarily a mixture of saturated and unsaturated hydrocarbons and alcohols such as methanol or tertiary alkylalcohols. The decomposition (conversion) reaction may be conducted in any suitable reactor which is packed with one or more beds of the alkaline earth exchanged catalyst. Prior to the introduction of feed, steam is fed to the reactor at conducted at a temperature from 100 to 400 °C (212 to 752° F), preferably from 100 to 232° C (212 to 450 °F), and most preferably to 121 to 177° C (250 to 350° F), at a space velocity of from 0.1 to 15 hr 1 WHSV, preferably from 0.3 to 5 hr 1 WSHV, and most preferably from 0.5 to 3 hr 1 WHSV, and at a pressure of from atmospheric pressure to 4140 kPag (600 psig), preferably from atmospheric pressure to 1720 kPag (250 psig), and most preferably 103 to 1030 kPag (15 to 150 psig). The flow is maintained until condensate is seen in the outlet of the reactor. At this point, the catalyst is saturated and steaming is discontinued. This same steaming method can be applied to other zeolite catalyst systems which have very good catalyst activity, but poor selectivity. Ether is then fed to the reactor at normal operating conditions. Reactor operating temperatures for this process are generally relatively low, preferably ranging from 51° to 315° C (125° to 600° F), more preferably from 115° to 260° C (240° to 500° F) and most preferably from 137° to 193° C (280° to 380° F). Operating pressure may range for example from atmospheric to about 1.72 MPag (250 psig), with 344 to 862 kPag (50 to 125 psig) being preferred. Pressure is preferably such that the reaction occurs substantially in the vapor phase. The reactor may be equipped with a suitable temperature controlling means such that the desired operating temperatures can be maintained or adjusted in the reactor. In a continuous process the reaction is preferably carried out at a spatial velocity expressed in terms of weight of organic feed per unit weight of catalyst per hour in the range of from 0.5 to 100 WHSV, more preferably from 1 to 20 WHSV. The process is especially suited for the conversion of fractions containing tertiary amylmethyl ether into corresponding isopentene olefins such as 2-methyl-2-butene or 2-methyl-l-butene; and for conversion of fractions containing methyl tertiary butyl ether into isobutylene. A particular advantage of the process is that the decomposition (conversion) product generally contains only a very low content of the corresponding by-product alkanes, such as isobutane or isopentane, which are very difficult to separate from their olefin counterparts. Even though the catalyst could be used without the steam pretreatment, the selectivity is enhanced and the by-product yields are reduced with this pretreatment step. By using the steam pretreatment, the by-product yields are reduced by at least 20%, preferably at least 30%, and most preferably at least 40%. By-products, include, for example, in the conversion reaction of tertiary butyl methyl ether to isobutylene, methyl secondary butyl ethers, dimethyl ether, and isobutane. The following examples illustrate the invention. Example 1 - CaY Catalyst Preparation Prior to Steam Pretreatment A calcium exchanged zeolite Y catalyst was prepared as follows: 108.3 grams of pellets of zeolite Y (LZY-52, available from UOP) which contained 20% by weight of alumina as a binder were packed into a 45.72 cm (18 inch) glass column. The column was then flushed with 100 ml. of ultra high purity water (pH-6.7) at a temperature of 65.5° C (150PF). A solution of 217 g of calcium chloride in 3500 ml. of ultra high purity water was formed and this solution was then passed through the packed zeolite bed at a rate of 2 ml. per minute at 65.5° C (150° F). The packed zeolite was then washed with additional pure water until the effluent was essentially free of chloride ions as indicated by a negative silver nitrate test. The exchanged zeolite was then dried overnight under a vacuum at ambient temperatures and then dried at 100° C (212° F) for 8 hours under vacuum. Analysis showed that about 90% by weight of the original sodium ions present in the zeolite had been exchanged by calcium ions. Example 2 - Comparative CaY Catalyst Performance Without Steaming The exchanged zeolite of Example 1 was crushed and sieved to 20-40 mesh and packed into a 30.48 cm by 0.635 cm (12 inch by 0.25 inch) stainless steel reactor column which was then connected to a feed line. The reactor was placed in a circulating hot air oven and also connected to an effluent collector line. A feed stream containing 95+ % of tertiary butyl methyl ether was preheated and passed into the inlet of the reactor at a constant temperature maintained at about 174.9° C (345° F), at a pressure of 620.5 kPag (90 psig) and at a WHSV in the range of from about 3 to 5. Reaction product removed from the discharge of the reactor showed an initial conversion rate of greater than 95% of tertiary butyl methyl ether to isobutylene. The process was continued under constant conditions of pressure and temperature until the % conversion to isobutylene dropped below 90%. Example 3 - Invention CaY Catalyst Performance With Steaming Example 2 was repeated under the conditions set forth therein except the catalyst was steamed prior to the introduction of the feed steam. The catalyst was steamed to saturation at WHSV =2, 174° C (345° F), and atmospheric pressure. Example 2 and 3 - Comparisons The results are illustrated in Figures 1-4 showing the effect of catalyst performance with and without steaming. For the purpose of comparison, a value was developed to chart the relative performance of each charge of catalyst under varying conditions. The value was normalized to the initial activity of fresh normal hydrogen floride (HF) treated attapulgite clay, as disclosed in US-A-4,691,073, at 360° F with tertiary butyl methyl ether. This value called the effective activity was empirically determined to be: A = C - ln(l - C/100) - t(T) + n C = % conversion or MTBE T = temperature in°F t = temperature normalization factor (delta C/delta T) n = constant to normalize to 100% The initial activity of the steam treated Ca-Y declined approximately 7 percent over an unsteamed catalyst, but the primary by-products of dimethyl ether (DME), butenes, and isobutenes were significantly reduced. DME production was reduced by 75 percent, methyl secondary butyl ether (MSBE) conversion (used as a measure of butene formation) was reduced 42 percent, and isobutane was reduced 100 percent (was not detectable < 10 ppm). The ether decomposition activity of the steam treated catalyst returned to the same level as the unsteamed catalyst after 75 hours of operation. From this point on, however, the same slope of deactivation was observed, but by¬ product formation was maintained at the greatly reduced level. Examples 4 and 5 - Comparative ZSM5 Catalyst Performance With and Without Steaming Examples 2 and 3 were repeated under the conditions set forth therein except the catalyst used was ZSM5. The catalyst was steamed to saturation at WHSV =2, 174° C (345° F), and atmospheric pressure. Example 4 and 5 - Comparisons The results are illustrated in Figures 5-8 showing the effect of ZSM5 catalyst performance with and without steaming. For the purpose of comparison, the effective activity was determined as denoted above. The initial activity of the steam treated ZSM5 declined approximately 3 percent over an unsteamed catalyst, but the primary by-products of dimethyl ether (DME), butenes, and isobutenes were significantly reduced. DME production was reduced by 59 percent, methyl secondary butyl ether (MSBE) conversion (used as a measure of butene formation) was reduced 19 percent, and isobutane was reduced 69 percent. The ether decomposition activity of the steam treated catalyst returned to the same level as the unsteamed catalyst after 7 hours of operation. From this point on, the same slope of deactivation was observed, but by-product formation was maintained at the greatly reduced level.

Claims: CLAIMS: 1. A process for selectively converting an alkyl ether to its corresponding olefin which comprises contacting the ether with a catalyst comprising a faujasite, wherein at least 50% by weight of the original alkali metal content of the catalyst has been exchanged with at least one alkaline earth metal comprising calcium, barium or strontium, characterized in that prior to the contacting the catalyst has been pretreated with steam. 2. The process of Claim 1 wherein the catalyst is zeolite Y. 3. The process of any preceding claim, wherein the catalyst has been pretreated with steam at a temperature in the range of from 100 to 400 °C (212 to 752 °F), and/or at pressure from atmospheric to 4140 kPag (600 psig), and/or at a space velocity of from 0.1 to 15 hr _ l WHSV. 4. The process of any preceding claim wherein the catalyst is saturated with steam. 5. The process of any preceding claim wherein the ether has from 5 to 12 carbon atoms, preferably from 5 to 8 carbon atoms. 6. The process of any preceding claim wherein the ether is a tertiary alkyl ether. 7. The process of Claim 6 wherein the ether comprises tertiary butyl methyl ether or tertiary amyl methyl ether. The process of any preceding claim wherein the selectivity of the alkyl ether to its corresponding olefin is such that the rate of conversion of ether to olefin is at least 60% by weight, preferably at least 90% by weight, most preferably at least 95% by weight. 9. The use of a faujasite catalyst, at least 50% by weight of the original alkali metal content of which has been exchanged with at least one alkaline earth metal and which has been pretreated with steam, for conversion of an alkyl ether to its corresponding olefin. 10. The use of a faujasite catalyst as starting material in a procedure which comprises exchanging at least 50% by weight of the original alkali metal content of the catalyst with at least one alkaline earth metal and pretreating the catalyst with steam to form a catalyst for use in conversion of an alkyl ether to its corresponding olefin. 11. The process of any preceding claim wherein the byproduct make from the conversion reaction is reduced by at least 20%, preferably at least 30%, as compared to that make using the catalyst which has not been pretreated with steam. 12. The process of Claim 7 wherein the by-product yields of methyl secondary butyl ethers, dimethyl ether, and isobutane are reduced by at least 20%, preferably at least 30%.

Inventors: COX GRAEME IAN, PETE DERRICK DUMONT, ROSENFELD DANIEL DAVID, COX, GRAEME, IAN, PETE, DERRICK, DUMONT, ROSENFELD, DANIEL, DAVID
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1994
Main classification: C07C11
IPC codes: B01J29, C07B61, C07C1, C07C11